linux-kernel - rw_semaphore performance, was: new metadata reader/writer locks in integration-test

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Date:	Fri, 22 Jul 2011 11:01:51 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	linux-btrfs <linux-btrfs@...r.kernel.org>,
	linux-kernel@...r.kernel.org, david@...morbit.com
Subject: rw_semaphore performance, was: new metadata reader/writer locks in
 integration-test

On Tue, Jul 19, 2011 at 01:30:22PM -0400, Chris Mason wrote:
> We've seen a number of benchmarks dominated by contention on the root
> node lock.  This changes our locks into a simple reader/writer lock.
> They are based on mutexes so that we still take advantage of the mutex
> adaptive spins for write locks (rwsemaphores were much slower).

Interesting.  Do you have set up some artifical benchmarks for this?

I wonder if the lack of adaptive spinning has something to do with the
slightly slower XFS performance on Joern's flash testing, given that
we extensively use the rw_semaphore as the primary I/O mutex, while
all others rely on plain mutexes as the primary synchronization
primitive.
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